Filtering materials are used for removing undesired material from a fluid, forcing the flow of fluid through the filter.
In the prior art, glass microfiber is widely used in filtration. Glass microfiber ensures a high mechanical, chemical and physical resistance with good filtering performance. The drawbacks that have always accompanied the use of this material are related to the non-incinerability and to the hazard to the health and safety of the people involved in the various production steps.
Polymer microfibers assembled to form multilayer filters have been proposed as an alternative. These filters obviate the environmental problem by excluding the use of fiberglass and maintaining acceptable filtering efficiency features.
However, a problem is strongly felt when using this type of product alternative to fiberglass: an accumulation capacity lower than that observed using fiberglass.
WO2007126621 describes a multilayer filtering system wherein the fibers are made of polybutylene naphthalate (PBN). In a first layer, the fibers have a diameter of between 0.5 and 8 microns, in a second layer between 10 and 35 microns.
US2001/0309012 also describes a polymer multilayer filtering system which comprises at least two layers: a loading layer, where the fibers have a diameter of 5-30 microns and a basic weight of 30-100 g/m2, and an efficiency layer where the fibers have a diameter of 0.5-3 microns and a basic weight of 40-150 g/m2. In said multilayer, the object is achieved by using two-component fibers, i.e. comprising a portion of thermoplastic fibers having a certain melting point and a second portion of fibers having a lower melting point.
While the two filters described above solve the environmental problem, they do not reach the filtering efficiency and the accumulation capacity reached by the use of fiberglass.
In addition, in the fluid filtration field, and in particular in the field of filtration of liquids such as oils and fuels, there are substantial problems related to the formation of electrostatic charges.
Flowing through a non-conductive filtering material, as a polymer, ceramic and/or fiberglass material typically is, oils and fuels promote the formation of charge accumulations due to triboelectric effect. The electrostatic charge thus accumulated causes discharges which damage the integrity, efficiency and correct operation of the filter and, under certain conditions, can result in explosive phenomena. The frequency and magnitude of these phenomena depend on the features of the non-polar liquid, such as chemical composition, viscosity, presence of contaminants, and also on the environmental conditions of use of the liquid, temperature, relative humidity, pressure and velocity, and on the features of the hydraulic system.
The filtering materials produced as multilayer of non-woven fabric made from insulating thermoplastic polymer materials have a high volumetric and surface resistivity which promotes the formation and the accumulation of electrostatic charges.
In order to overcome this problem, U.S. Pat. No. 6,099,726 describes the introduction of metal meshes into the filtering system.
US 2009/0078637 describes metal or chemical surface coatings.
US 2011/0005990 describes surface treatments.
The object of the present invention is to provide a fully incinerable filtering system, i.e. entirely devoid of fiberglass, with high mechanical, chemical and physical resistance, as well as with antistatic properties and a high accumulation power with respect to the prior art for polymer filters having equal filtering efficiency.